The present invention relates to the production of ceramic media, and particular media useful in mass transfer applications in which the media are subjected to repeated thermal cycling. An important example of the fields in which the improved media are useful is in conjunction with thermal regenerative oxidizers (RTOs). Conventional media subjected to thermal cycling undergo volume dimensional changes during thermal cycling and this can lead to weakness and eventually loss of physical integrity, particularly if subjected to abrasive forces, or static forces from the load of media in a thermal oxidizer or other industrial process column. Media with reduced susceptibility to dimensional changes during thermal cycling would therefore have a significant advantage.
A further problem with conventional ceramic media is that they inherently suffer large volumetric changes during firing. Such media are conventionally made by blending suitable ceramic-forming raw materials such as clay, feldspar, talc, wollastonite, zircon sand and other minerals. This mixture is then formed into a shapeable mixture and formed into so-called xe2x80x9cgreenwarexe2x80x9d which has the shape of the desired ceramic media but not the physical properties and is converted into the media by firing the greenware at an elevated temperature. During the firing, the components interact through a combination of solid state, liquid phase, and vapor phase sintering mechanisms to form the desired ceramic material. The problem is that, when the shaped greenware is fired, a volume change usually occurs that can easily be from 25 to 35%. Firing occurs in a kiln which is of limited volume. Thus it can only produce a volume of ceramic product that is xc2xe to ⅔ of the volume of greenware loaded. This makes the process very inefficient.
A formulation and a firing process have now been developed that result in a ceramic with little volume change or deterioration of physical properties as result of thermal cycling and little or no productivity loss as a result of loss of product volume during firing.
The present invention provides ceramic media obtained by firing greenware shaped from a mixture comprising 5 to 60 wt. % and preferably from 5 to 20 wt % of spodumene, and from 95 to 40, and preferably from 95 to 80 wt. % of other ceramic-forming components comprising from 50 to 95 wt. % of clay and from 5 to 50 wt. % of feldspar based on the combined weights of these components, said media having a linear shrinkage of less than 5%, and preferably less than 2.5%, by comparison with the dimensions of the pre-fired greenware
The invention further comprises a process for the production of ceramic media which comprises:
a) forming a mixture comprising from 5 to 60, and preferably from 5 to 20, wt % of spodumene and from 95 to 40, and preferably from 95 to 80 wt. % of other ceramic-forming components comprising from 50 to 95 wt. % of clay and from 5 to 50 wt. % of feldspar based on the combined weights of these components all weights being based on the total weight of ceramic-forming components, into greenware having a desired shape; and
b) firing the greenware to produce ceramic media;
wherein the ceramic-forming components are chosen such that, upon firing, the linear dimensions of the greenware are not reduced by more than 5% and more preferably by not more than 2%.
Particularly preferred mixtures comprise 5 to 20 and more preferably 10 to 15 wt. % of spodumene; and 95 to 80 and more preferably 90 to 85 wt % of other ceramic-forming components of which from 30 to 95 wt. % and preferably from 50 to 90 wt % is a kaolinite clay; and from 5 to 70, and preferably 10 to 50 wt. % is a feldspar, all percentages being based on the total weight of the other ceramic-forming components.
Spodumene is a naturally occurring mineral that is a mixture of the aluminosilicates of lithium and sodium and two of the common minerals are xe2x80x9ckuntzitexe2x80x9d and xe2x80x9chiddenitexe2x80x9d. The generic name is xe2x80x9ctriphanexe2x80x9d. The preferred dominant mineral is a lithium aluminosilicate and the lithium content of the mineral, measured as lithium oxide, is between about 7.15 and 7.75% by weight.
Clays are generically mixed oxides of alumina and silica and include materials such as kaolin, ball clay, fire clay, china clay and the like. Preferred clays are high plasticity clays such as ball clay and fire clay. Particularly preferred clays have a methylene blue index, (xe2x80x9cMBIxe2x80x9d), of about 11 to 13 meq/100 gm.
The term xe2x80x9cfeldsparsxe2x80x9d is used herein to describe silicates of alumina with soda, potash and lime.
Other components such quartz, zircon sand, feldspathic clay, montmorillonite, nepheline syenite and the like can also be present in minor amounts of the other ceramic-forming components in the formulations according to the invention providing the resultant formulation meets the dimensional change requirements set forth above.
The components fired together to produce the ceramic products of the invention are preferably supplied in fine powder form and are made into a shapeable mixture by the addition of water and or extrusion aids. Shaping can be by molding but economics usually dictate that the shapes are made by an initial extrusion process followed by cutting perpendicular to the direction of extrusion into the desired lengths.
The fired ceramic media have an apparent open porosity, as measured by ASTM C-373, that is less than 8% by volume and preferably less than 4% by volume. The amount of absorbed water is less than 4% by weight and more preferably less than 2% by weight, as measured according to the method described in ASTM-C-373.
The process by which the ceramic media can be made includes mixing the components with a mixing medium such as water and shaping the mixture into the desired shape such as by extrusion or molding to form greenware, and then drying the greenware at a temperature sufficient to drive off associated water over a period that is preferably several hours. This is to avoid disrupting the weak structure of the greenware and is usually carried out at below about 120xc2x0 C. and frequently below about 70xc2x0 C. and lasts for about 5 hours. The dried greenware is then fired at an elevated temperature, such as from 1100 to 1300xc2x0 C., which is reached gradually over a period of from about 3 to about 20 hours and is usually maintained for from 1 to 5 hours before allowing a gradual cooling to ambient temperatures.
The ceramic media of the invention can have the form of monoliths with multiple through passages but more appropriately the media are in the form of random or dumped packings with shapes such as rings, cylinders, spheres, pellets and the like. Media in such shapes are placed in a heat exchange unit in random fashion with as many incorporated as the space can accommodate.
The ceramic media according to the invention have an added unexpected advantage in that they typically lose less than 10% of crush strength and preferably less than 5% after having been subjected to a temperature cycle of 800xc2x0 C. Indeed they often seem to increase their crush strength.
The invention is now described with specific reference to the following Examples, which are intended to illustrate but not delimit aspects of the invention.
The materials used in the Examples were as follows: